This invention relates to ellipsometry.
One class of ellipsometers includes a light source, a polarizer, a modulator, an analyzer and at least one intensity detector. In this prior art type of device, the light is modulated and sensed by a photocell. The resulting electric signal is used to calculate psi and delta, which can be done by a number of known techniques including a Fourier expansion of the resulting signal.
In a prior art type of ellipsometer in this class, called a large modulation ellipsometer, the modulator varies the intensity of the beam over a defined range sufficiently large to give an easily measured signal.
This type of ellipsometry has a disadvantage in that the system must be calibrated to take into account nonlinear changes in output signals dependent upon light intensity or polarization state. For example, the photocell output signal in some ranges changes nonlinearly with respect to changes in the intensity of light and the light reflected from diffration gratings, if any are included, change nonlinearly with respect to changes in the phase. The changes in ranges of intensity and polarization state are caused by changes in reflected or transmitted light as the environment of measurement changes.
Because of the need for recalibration with changes in the environment, corrections may need to be made such as in the software to make the necessary corrections or adjustment of voltages or adjustment of amplification level of amplifiers or sensors or attenuation levels of signals from photocells or changes in the angle of the diffraction grating if one is included or in the polarizer. This alignment is generally obtained by measuring the psi and delta of a known environment similar to the one being tested and calibrating the instrument in that known environment.
This disadvantage of prior art type of ellipsometers is especially severe when continuous measurements of time varying quantities are made because of the need to calibrate for different surfaces.